The invention relates generally to braking in turbines and, in particular, to hybrid braking in wind turbines.
Wind turbines have been used as a source of power for many years. Among the various operational parameters, speed control and emergency braking capability have particularly strong influences on the structural stability and longevity of the components within the wind turbine. Controlling the speed of a wind turbine rotor below its maximum limit is necessary in order to avoid the damaging effects of high velocity winds on the wind turbine and its supporting structure. Structural damage due to over-stressing the turbine blades and rotor can occur if the rotational speed of the wind turbine is allowed to increase without limit.
Typically, mechanical braking systems in wind turbines are configured to halt the turbines during an emergency or any other event that requires stopping the turbine. However, dynamic control and fast response are difficult to achieve in mechanical braking during operation of the turbine. Another approach includes deploying hydraulics to aero-brake devices wherein the blade tips are moved to counter the wind force during braking. In yet another approach, a mechanical braking scheme is implemented that allows the rotor blades to deflect backwards and in the direction of the wind flow under gusting wind conditions. This in turn alters the rotor blade pitch or angle of incidence to respond to wind conditions. To prevent damage or destruction to the wind turbine and other wind turbines when within a wind farm, a braking system must provide reliable braking and back-up braking systems.
However, such mechanical braking approaches as discussed above have certain disadvantages. All of these devices add significant cost and weight to the rotor blades and the overall drive train. Aero-brakes implementing dynamic wing tip brakes and associated hardware have low tolerance limits and increase design, manufacturing, and maintenance costs. Further, added weight increases the strain on the braking system. By implementing only mechanical braking during an emergency stop, dynamic torque components cannot be avoided. To handle such dynamic torque, the mechanical drive train has to be designed with adequate safety margins that add weight to the wind turbine structure. Furthermore, such mechanical brakes induce fatigue on gearboxes due to transient overload and dynamic torque components during emergency stops.
Therefore there is a need for an improved braking system.